Conventionally, in a surface protecting film or interlayer inslating film of a semiconductor device, a polyimide resin having excellent heat resistance and high electric and mechanical properties has been used. In general, a film of the polyimide resin is formed by reacting a tetracarboxylic acid dianhydride with a diamine in a polar solvent at room temperature under atmospheric pressure, applying the resultant polyimide precursor (polyamidic acid) solution (so-called varnish) by spin coating or the like to form a thin film, and subjecting the thin film to thermal cyclodehydration (hardening) (see, for example, Nonpatent Document 1).
In recent years, a photosensitive polyimide obtained by imparting photosensitive properties to the polyimide resin has been used. The use of the photosensitive polyimide has a characteristic in that a patterning step can be simplified, making it possible to shorten a cumbersome process for pattern formation (see, for example, Patent Documents 1 to 3).
In development process of the photosensitive polyimide, an organic solvent such as N-methylpyrrolidone has conventionally been used. However, recently, from the viewpoint of environmental burden and cost, there has been proposed a positive photosensitive resin that can be developed using an alkaline aqueous solution. As examples of methods for obtaining a positive photosensitive resin that can be developed using an alkaline aqueous solution, there can be mentioned a method in which an o-nitrobenzyl group is introduced to a polyimide precursor via an ester linkage (see, for example, Nonpatent Document 2), and a method in which a naphthoquinonediazide compound is mixed into soluble hydroxylimide or a polybenzoxazole precursor (see, for example, Patent Documents 4 and 5). Resins obtained by the aforementioned methods can be expected to achieve reduced permittivity, and also from this viewpoint, the photosensitive polybenzoxazole has attracted attention as well as the photosensitive polyimide.
Recently, as a process for forming a resin hardened film, such as an interlayer inslating-film layer or surface-protecting film layer for an electronic part, a low temperature process is desired. In order to meet such a demand, a polyimide or polybenzoxazole therefor is required to have capability of being cyclodehydrated at a low temperature and capability of giving properties to the resulting film after the cyclodehydration comparable to the properties of a film obtained by cyclodehydration at a high temperature. However, when the polyimide precursor or polybenzoxazole precursor is subjected to thermal cyclodehydration to form a polyimide thin film or a polybenzoxazole thin film, the cyclodehydration generally needs a temperature as high as about 350° C. Such a high temperature of about 350° C. possibly adversely affects the substrate. In order to avoid defects caused due to the heat history, it has recently been desired to lower the temperature for treatment in a semiconductor manufacturing process. In order to achieve a lowered treatment temperature in the process, even with respect to the surface protecting film, a polyimide material or polybenzoxazole material therefor is required to have capability of being cyclodehydrated at a temperature as low as less than about 250° C., which is lower than the high temperature of about 350° C. conventionally employed, and capability of giving properties to the resulting film after the cyclodehydration comparable to those of a film obtained by cyclodehydration at a high temperature. However, when the cyclodehydration is performed using a thermal diffusion oven at a low temperature, the resultant film generally has poor properties.
It is known that the cyclodehydration temperature for a polybenzoxazole precursor is generally higher than the cyclodehydration temperature for a polyimide precursor (see, for example, J. Photopolym. Sci. Technol., vol. 17, pp. 207-213). Therefore, cyclodehydration of the polybenzoxazole precursor at a temperature of lower than 250° C. is more difficult than cyclodehydration of the polyimide precursor.    Patent Document 1: Japanese Patent Application Laid-open No. S49-11551 A    Patent Document 2: Japanese Patent Application Laid-open No. S59-108031 A    Patent Document 3: Japanese Patent Application Laid-open No. S59-219330 A    Patent Document 4: Japanese Patent Application Laid-open No. S64-60630 A    Patent Document 5: U.S. Pat. No. 4,395,482    Nonpatent Document 1: Nippon Polyimide Kenkyukai (Japan Polyimide Study Group) “Saishin Polyimide—Kiso to Ouyou—(Latest polyimide—Basics and Applications—)” (2002)    Nonpatent Document 2: J. Macromol. Sci., Chem., vol. A24, 12, 1407 (1987)